U.S. patent number 4,198,115 [Application Number 05/934,172] was granted by the patent office on 1980-04-15 for fabry-perot resonator using a birefringent crystal.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to Ivan P. Kaminow.
United States Patent |
4,198,115 |
Kaminow |
April 15, 1980 |
**Please see images for:
( Certificate of Correction ) ** |
Fabry-Perot resonator using a birefringent crystal
Abstract
The cleavage planes of a lithium niobate crystal (100) are used
to provide the parallel surfaces (121 and 122) of a Fabry-Perot
resonator established in a titanium diffused waveguide (110). To
enable use of the extraordinary wave and therefore to provide a
higher efficiency resonator, the waveguide is diffused into the
crystal at a predetermined oblique angle (.PSI.) with respect to
the cleaved ends. The resonator is utilized in an optically pumped
laser and in an electrically controllable Fabry-Perot resonator
wherein the reflection provided by the cleaved ends is supplemented
by multilayered dielectric mirrors (201 and 202 or 301 and 302)
that are bonded to each of the cleaved ends. In the electrically
controllable resonator a dielectric layer (330) is deposited over
the surface of the crystal before electrodes (310 and 311) are
formed on the layer parallel to the waveguide.
Inventors: |
Kaminow; Ivan P. (Tinton Falls,
NJ) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, NJ)
|
Family
ID: |
25465085 |
Appl.
No.: |
05/934,172 |
Filed: |
August 16, 1978 |
Current U.S.
Class: |
385/11; 372/105;
385/132; 372/108 |
Current CPC
Class: |
G02F
1/225 (20130101); G02F 1/213 (20210101); G02F
1/035 (20130101) |
Current International
Class: |
G02F
1/225 (20060101); G02F 1/01 (20060101); G02F
1/035 (20060101); G02F 1/21 (20060101); G02B
005/14 () |
Field of
Search: |
;350/96.12,96.13,96.14,355,356 ;331/94.5C,94.5F |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Smith et al., "Integrated Bistable Optical Devices," Appl. Phys.
Ltrs., vol. 33, No. 1, Jul. 1978, pp. 24-26. .
Smith et al., "Electrooptic Nonlinear Fabry-Perot Devices," IEEE
Journal of Quantum Electr., vol. QE-14, No. 3, Mar. 1978, pp.
207-212. .
Hsu et al., "Flip-Chip Approach to Endfire Coupling . . . ,"
Electronics Letters, vol. 12, No. 16, Aug. 1976, pp. 404-405. .
Steinberg et al., "Modal Fields of Anisotropic Channel Waveguides,"
J.O.S.A., vol. 67, No. 4, Apr. 1977, pp. 523-533. .
Findakly et al., "Optical Directional Couplers with Variable
Spacing," Applied Optics, vol. 17, No. 5, Mar. 1978, pp. 769-773.
.
Kaminow et al., "Loss in Cleaved Ti-Diffused LiNbO.sub.3
Waveguides," Appl. Phys. Ltrs., vol. 33, No. 1, Jul. 1978, pp.
62-64. .
Burns et al., "Mode Dispersion in Uniaxial Optical Waveguides,"
J.O.S.A., vol. 64, No. 4, Apr. 1974, pp. 441-445. .
Kaminow et al., "Nd:LiNbO.sub.3 Laser," IEEE Journal of Quantum
Elect., vol. QE-11, No. 6, Jun. 1975, pp. 306-308. .
Schmidt et al., "Metal-Diffused Optical Waveguides in LiNbO.sub.3
", Appl. Phys. Ltrs., vol. 25, No. 8, Oct. 1974, pp. 458-460. .
Kaminow et al., "Efficient Strip-Waveguide Modulator," Appl. Phys.
Ltrs., vol. 27, No. 10, Nov. 1975, pp. 555-557..
|
Primary Examiner: Corbin; John K.
Assistant Examiner: Lee; John D.
Attorney, Agent or Firm: Dubosky; Daniel D.
Claims
I claim:
1. A Fabry-Perot resonator comprising a birefringent crystal (100)
having at least two parallel cleaved ends (121 and 122), and a
strip waveguide (110) in said crystal between said two ends,
characterized in that, said strip waveguide is oriented in said
crystal so as to intercept said cleaved ends at an oblique
angle.
2. A Fabry-Perot resonator as defined in claim 1 wherein said
crystal is lithium niobate and said strip waveguide is established
in said crystal by the presence of diffused titanium.
3. A Fabry-Perot resonator as defined in claim 1 wherein the
resonator further includes two mirrors (201 and 202) each one of
which is bonded to one of said two parallel cleaved ends.
4. A Fabry-Perot resonator as defined in claim 3 wherein said
mirrors are multilayered dielectric mirrors.
5. A Fabry-Perot resonator as defined in claim 3 wherein said
crystal is composed of lithium niobate doped with neodymium.
6. A Fabry-Perot resonator as defined in claim 3 wherein said
resonator further includes a dielectric layer (330) overcoating the
surface of said crystal that contains said waveguide, and at least
two metal electrodes (310 and 311) deposited on said dielectric
layer near said waveguide and extending parallel to said waveguide
by substantially the entire length of said waveguide.
7. A Fabry-Perot resonator comprising a birefrigent crystal (100)
having at least two parallel cleaved ends and a c axis that lies in
a plane parallel to the top surface of said crystal, and a strip
waveguide (110) in the top surface of said crystal, characterized
in that, said strip waveguide is oriented with its axis at an angle
of .PSI. with respect to a perpendicular drawn to either one of sid
two parallel cleaved ends, such that the magnitude of .PSI. is
greater than zero.
8. A Fabry-Perot resonator as defined in claim 7 wherein the c axis
is at an angle .theta. with respect to one of said two parallel
cleaved ends and at an angle .alpha. with respect to the axis of
said waveguide, the angle .PSI. is substantially defined by the
following equations:
where
and
where .epsilon..sub.xz and .epsilon..sub.xx are elements of the
dielectric tensor in the coordinate system where z is parallel to
the waveguide axis, n.sub.3 is the index of refraction of said
crystal for polarization parallel to the c axis, and n.sub.1 is the
index of refraction of said crystal for polarization perpendicular
to the c axis.
9. A Fabry-Perot resonator as defined in claim 7 wherein the
resonator further includes two mirrored surfaces (201 and 202) each
one of which is bonded to one end of said cleaved ends.
10. A Fabry-Perot resonator as defined in claim 9 wherein the
resonator further includes a dielectric laye (330) overcoating the
top surface of said crystal and at least two electrodes (310 and
311) deposited on the surfaces of said dielectric layer
substantially parallel to said waveguide and extending for
substantially the entire length of said waveguide.
Description
TECHNICAL FIELD
This invention relates to Fabry-Perot resonators and more
particularly, to Fabry-Perot resonators that use a birefringent or
uniaxial crystal as the resonator medium.
BACKGROUND OF THE INVENTION
Electrically controlled Fabry-Perot resonators have recently found
use in a wide variety of optical devices. See the article
"Electrooptic Nonlinear Fabry-Perot Devices", by P. W. Smith et al,
IEEE Journal of Quantum Electronics, Vol. QE-14, No. 3, March 1978,
pages 207-212. The device disclosed in this Smith et al article can
perform a wide variety of functions including optical limiting,
pulse shaping, and amplification, and be used as a switching and
logic element. It is suggested on page 208 of the Smith et al
article that an integrated optics version can be constructed by
using an electro optic modulator incorporating a titanium diffused
waveguide in a lithium niobate substrate as the resonator medium.
To construct such a device requires that the end faces of the
titanium diffused waveguide be flat and substantially perpendicular
to the direction of propagation in the waveguide. To achieve this
flat surface by the standard techniques of cutting and polishing
the end of the crystal is extremely difficult since rounding of the
edges almost always occurs with polishing, and the titanium
diffused waveguide emerges at one edge of the crystal.
It is well known in the art that cleavage planes in the crystal can
be utilized to provide reproducable and rectangular ends without
difficulty. See for example, the article entitled "Flip-Chip
Approach to Endfire Coupling Between Single Mode Optical Fibres and
Channel Waveguides", by H. P. Hsu and A. F. Milton, Electronics
Letters, Vol. 12, No. 16 pages 404-405, Aug. 5, 1976. Hence, a
Fabry-Perot resonator for use in the Smith et al type devices can
be constructed by using a lithium niobate crystal with cleaved ends
as the parallel end faces of the resonator. Since lithium niobate
has an index of refraction that is not radially different from that
of air, an external mirror may be required at each of the cleaved
ends to provide the necessary degree of reflection.
As pointed out in the Smith et al article, it is highly desirable
to have as high a degree of sensitivity as possible in the
electrically controllable Fabry-Perot resonator. That is, it is
desirable to have a resonator medium which produces as large a
phase shift as possible for any given change in voltage across the
electrodes of the electrically controllable Fabry-Perot
resonator.
SUMMARY OF THE INVENTION
One method of achieving increased electro-optic sensitivity in the
Fabry-Perot resonator is to use the extraordinary wave for
propagation in metal diffused waveguides that have been fabricated
in certain birefringent crystals. The cleaving method, however, of
achieving parallel surfaces in the waveguide requires a special
crystal orientation. In lithium niobate, for example, use of the
cleavage plane to achieve the parallel surfaces results in a
crystal whose unique axis lies in a plane that is parallel to the
top surface of the waveguide. Hence, propagation in the
extraordinary wave requires that the radiation be polarized in a
plane that is parallel to the top surface of the waveguide. If the
strip waveguide is oriented so as to be perpendicular to the end
faces or cleavage planes, orientation of the incoming radiations so
as to position the wave normal vector perpendicular to the face
results in a Poynting vector that is not in line with the waveguide
axis. If, on the other hand, the incoming radiation is oriented so
as to position the Poynting vector along the axis of the waveguide,
the wave normal vector is no longer perpendicular to the end faces
and the required standing wave pattern will not be set up in the
Fabrey-Perot resonator.
Propagation of the extraordinary wave in a Fabry-Perot resonator
using a birefringent crystal as the resonator medium and cleaved
ends as the parallel surfaces of the resonator is achieved in
accordance with the present invention by positioning the metal
diffused waveguide in the medium at an oblique angle with respect
to the cleaved ends of the crystal. Mirrors can be deposited on or
cemented to each one of the cleaved end faces in order to increase
the reflectivity. In lithium niobate, a titanium diffused waveguide
is positioned in the crystal at an angle which is approximately two
degrees from the vector that is normal to the cleaved end. As a
result, use of this Fabry-Perot resonator as an electrically
controlled Fabry-Perot resonator achieves an electro-optic effect
that is about 3.1 times greater than a Fabry-Perot resonator that
uses an ordinary wave in a waveguide that is normal to the cleaved
ends. The required switching energy for such a resonator in a
bistable device of the type disclosed in the Smith et al article is
about 10 times less than a resonator that uses the ordinary
wave.
In accordance with another aspect of the present invention, a
dielectric buffer layer can be deposited over the crystal surface
and metal electrodes are deposited on the dielectric layer along
paths that are parallel to the metal diffused waveguide. By
cementing mirrors on each cleaved end of the crystal an
electrically controllable Fabry-Perot resonator is provided and may
be used in turn to provide integrated optics versions of the many
optical devices disclosed in the above-identified Smith et al
article.
In accordance with still another aspect of the present invention
multilayered dielectric mirrors are cemented to each cleaved end of
the birefringent crystal to produce a laser that can be optically
pumped by radiation that is coupled into the metal diffused
waveguide. For some birefringent crystals such as neodymium doped
LiNbO.sub.3, the laser gain is greater for the extraordinary wave
than for the ordinary wave.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagram of a Fabry-Perot resonator constructed in
accordance with the present invention;
FIG. 2 is a pictorial illustration of a laser that uses the present
invention;
FIG. 3 is a pictorial illustration of an electrically controllable
Fabry-Perot resonator using the present invention;
FIG. 4 is a cross-sectional diagram of the device shown in FIG. 3;
and
FIGS. 5 and 6 are pictorial illustrations of the end faces of a
lithium niobate crystal, which illustrations are useful in
describing how to make a device using the present invention.
DETAILED DESCRIPTION
In order to construct an embodiment of the present invention, it is
first necessary to obtain a birefringent crystal such as lithium
niobate, or lithium tantalate. The crystal should then be cut to
the desired thickness and width and these cuttings must be oriented
with respect to the crystal so as to present the cleavage plane at
the ends of the crystals. The crystals can then be cleaved at each
end in order to provide the proper length of Fabry-Perot resonator.
To cleave the crystals, a scribe mark is made at one edge of the
crystal plate. Two pairs of tweezers are then used on either side
of the scribe mark in order to make the break. As is usual on
cleaved surfaces, a number of terraces appear on the cleaved edge.
It has been experimentally determined that the characteristic of
the terraces and the nature of the cleaved end face depend upon
whether the break starts near the negative or positive end of the c
or unique axis of the crystal. See the article entitled "Loss in
Cleaved Ti-Diffused LiNbO.sub.3 Waveguides" by I. P. Kaminow and L.
W. Stulz, Applied Physics Letters, Vol. 33, No. 1, July 1, 1978,
pages 62-64.
As indicated in FIG. 5 of the drawings, the c axis for a lithium
niobate crystal is in a plane parallel to the top surface of the
crystal at an angle of .theta. (equal to 32.75 degrees) from the
[10.2] cleavage plane. If the scribe mark is placed near the
negative end of the c axis as indicated in FIG. 5, a single series
of terraces appears on the end face of the crystal. If, on the
other hand, the scribe mark is placed near the positive end of the
c axis as indicated in FIG. 6, a double series of terraces appears
on the end face of the crystal. Using a reflection interference
microscope it has been observed that the cleaved end having the
double series of terraces as in FIG. 6 presents a nearly flat
surface with many terraces having small step heights less than a
few hundred A. If, on the other hand, the break starts near the
negative end of the c axis as in FIG. 5, the cleaved end with
single series of terraces presents a face that is often conchoidal
and contains few terraces. To obtain the flat end surface which is
necessary in a Fabry-Perot resonator, the break which provides a
double series of terraces has always been found to be superior.
After the proper dimensions have been established in the lithium
niobate crystal with cleaved ends, a waveguide can then be
established in the top surface of the crystal by diffusing a metal
such as titanium along a predetermined width of the top surface of
the crystal. In the embodiment constructed, a waveguide was
established in a 4-centimeter long crystal by diffusing a
4-micrometer wide and 180 A thick titanium strip for 5 hours at
1050 degrees Centigrade in the presence of flowing oxygen. The
crystal used was a standard acoustic graded lithium niobate
substrate obtained from Crystal Technology Incorporated. The 99.9
percent pure titanium was evaporated from a tungsten coil.
If it is desired to generate an ordinary wave in the waveguide, the
light that is coupled to the crystal can be polarized normal to the
crystal plate, that is, parallel to the a axis and perpendicular to
the c axis, and the guide can be fabricated in the crystal
perpendicular to the cleaved ends. See the article entitled
"Integrated Bistable Optical Devices" by P. W. Smith, I. P.
Kaminow, P. J. Maloney and L. W. Stulz, Applied Physics Letters,
Vol. 33, No. 1, July 1, 1978, pages 24-26. since the index of
refraction of lithium niobate is relatively close to the refractive
index of air, it is usually necessary to provide some additional
reflection in addition to that which is provided by the cleaved
ends. Metal mirrors can be deposited on the end faces or, as
indicated in the embodiment to be discussed hereinbelow, this
additional reflection can be advantageously provided by a
multilayered dielectric mirror of the type commonly used in gas
laser systems. If, on the other hand, a birefringent crystal other
than lithium niobate or lithium tantalate is used having a
refractive index much different from that of air, the cleaved ends
by themselves may provide adequate reflection for the desired
Fabry-Perot resonator.
In order to achieve a strong electro-optic effect, it is desirable
to establish in extraordinary wave in the metal diffused waveguide
of the birefringent crystal. Such a wave cannot be established in a
LiNbO.sub.3 birefringent crystal Fabry-Perot resonator where the
guide is positioned perpendicular to the cleaved ends. If the
incoming light is oriented to be perpendicular to the cleaved end
and polarized in the plane of the crystal plate to establish an
extraordinary wave, the Poynting vector will not be parallel to the
waveguide axis. To make the Poynting vector parallel to the
waveguide axis the incident beam must be oriented so as to enter at
about 4.5 degrees from the normal. Under the latter circumstances
the Poynting vector can be made parallel to the guide axis, but the
wave normal vector is then about 2 degrees from the guide axis and
the wave fronts are no longer parallel to the cleaved faces. Under
these circumstances no Fabry-Perot maxima or minima are
observed.
In accordance with the present invention, the desired maxima and
minima of a Fabry-Perot resonator are achieved in combination with
a Poynting vector that is parallel to the waveguide axis by
positioning the waveguide at an oblique angle with respect to the
cleaved ends. In FIG. 1 the top view of a birefringent crystal 100
is shown where cleaved ends 121 and 122 are established
perpendicular to the plane of the drawing at an angle of .theta.
with respect to the c or unique axis of the birefrigent crystal
100. A metal diffused waveguide 110 made of diffused titanium metal
is established in the birefringent crystal at an angle of .PSI.
with respect to a vector that is normal to the cleaved end 121. The
angle between the c axis and the waveguide axis is designated as
.alpha. in FIG. 1. For any input beam 130 of optical radiation that
is positioned perpendicular to cleaved end 121 and oriented so as
to enter waveguide 110, an extraordinary wave can be established in
the waveguide whose Poynting vector S is coincident with the
waveguide axis and whose wave normal vector k is perpendicular to
the cleaved end by satisfying the following equations:
where
and
Equation 1 is simply an expression of the relationship between the
angles illustrated in FIG. 1. Equation 2 determines the value of
.PSI. in terms of the elements of the dielectric tensor
.epsilon..sub.xz and .epsilon..sub.xx in the coordinate system
where z is coincident with the waveguide axis and Poynting vector,
and x is in the plane of the crystal. This latter xyz coordinate
system is illustrated in FIG. 1 by vectors 141 and 142. Equations 3
and 4 are expressions for the elements of the dielectric tensor
.epsilon..sub.xz and .epsilon..sub.xx, respectively, in terms of
.alpha., the angle between the c axis of the birefringent crystal
and the waveguide axis, n.sub.3 is the index of refraction for
polarization parallel to the c axis, and n.sub.1 is the index of
refraction for polarization perpendicular to the c axis. Equations
1-4 provide a set of transcendental equations that may be solved in
an iterative fashion to provide a self-consistent solution for any
birefringent crystal. If lithium niobate is used as the
birefringent crystal 100, .theta. is equal to 32.75 degrees and
n.sub.1 equals 2.27 and n.sub.3 equals 2.19 at a wavelength of 0.7
micrometers. For these parameters the above equations when solved
provide a value of .PSI. equal to about 2.0 degrees. Hence, for
lithium niobate, if the titanium diffused waveguide is established
in the birefringent crystal at an angle of about 2 degrees from a
normal to the cleaved end, beam 130 having a wavelength of 0.7
micrometers can be polarized in the plane of the crystal and an
extraordinary wave is established in waveguide 110 whose Poynting
vector is coincident with the waveguide axis and whose wave normal
vector is perpendicular to the cleaved ends. In such a crystal of
lithium niobate the electro-optic effect has been determined to be
about 3.1 times greater than for the ordinary wave.
FIG. 2 illustrates an optically pumped laser that can be
constructed in accordance with the principles of the present
invention. A uniaxial crystal 200, such as lithium niobate or
lithium tantalate that has been doped with a 1 percent
concentration of neodymium, is cut to the proper thickness and
width and cleaved to the proper length as discussed hereinabove in
connection with FIG. 1. A waveguide 210 is established in crystal
200 by diffusing titanium along a waveguide axis that is about 2
degrees with respect to a normal to the cleaved end. A multilayered
dielectric mirror 201 is cemented at one end of the crystal 200 and
a second multilayered dielectric mirror 202 is cemented at the
other end of crystal 200. Mirror 201 is constructed so as to pass
the radiation at 7525 A from a krypton laser 220 and to be as
highly reflective as possible to radiation at a wavelength of 1.08
micrometers. Mirror 202 on the other hand is constructed so as to
be as highly reflective as possible to the wavelength of 7525 A and
to be only partially reflective to the wavelength of 1.08
micrometers. This lithium niobate laser when pumped by a krypton
laser at a wavelength of 7525 A oscillates at about 1.08
micrometers with an extraordinary wave propagating in waveguide
210.
The invention can also be practiced in an electrically controlled
Fabry-Perot interferometer of the type that can be used in the
devices disclosed in the above-identified article by Smith et al.
To construct this device a lithium niobate crystal with a titanium
diffused waveguide identical to that shown in FIG. 1 is first
constructed. As indicated in FIG. 3, the top surface of the
birefringent crystal 100 is then completely overcoated with a 2000
A thick layer 330 of aluminum oxide which is placed on the top
surface of the crystal by sputtering aluminum oxide in a 1-to-1
oxygen and argon atmosphere. Layer 330 is then further oxidized to
reduce its discoloration by heating it in oxygen at 500 degrees
Centigrade for 2 hours. Metal electrodes 310 and 311 consisting of
300 A of titanium and 700 A of silver are then evaporated on to the
dielectric layer 330 at positions that are over or near to
waveguide 110 in crystal 100. The position of the electrodes with
respect to the dielectric layer 330 and waveguide 110 is also
illustrated in FIG. 4, which is a cross-sectional diagram of
cross-section 4--4' in FIG. 3. Each electrode is 20 micrometers
wide and the electrodes are spaced by about 9 micrometers. By using
the dielectric layer 330 the influence of the metal electrodes on
the wave propagation in waveguide 110 is considerably reduced.
Multilayer dielectric mirrors 301 ad 302 are then oriented and
cemented at each cleaved end of crystal 100 in order to provide
additional reflection to the radiation introduced by beam 350.
What has been described hereinabove are merely illustrative
embodiments of the present invention. Numerous departures may be
made by those skilled in the art without departing from the spirit
and scope of the present invention. For example, any other
birefringent crystal may be utilized and if its index of refraction
is sufficiently different from that of air, the multilayered
dielectric mirrors may be completely eliminated. In addition, other
type mirrored surfaces may be utilized to provide the additional
reflection at the cleaved ends of a lithium niobate or lithium
tantalate crystal. For example, silver may be disposed directly on
to the cleaved ends since these ends have a high degree of
parallelism. When using a silvered or aluminum reflecting surface,
it may be necessary to provide a space between the ends of
electrodes 310 and 311 and the cleaved ends of crystal 100 in order
to prevent any shorting of the potential that is applied to the
electrodes.
* * * * *